Once in a day is Ok for fish. be careful when you fed them though, this is because fish won't finish the entire food and the uneaten food normally settles down at the bottom gravel and gets rotten. This may alter the PH value of your tropical fish water and is harmful for the fish health.

Where can I buy the best quality tropical fish food?

Normally pet stores or even aquarium stores sell good quality tropical fish food in the form of plastic packets or bottles. When you buy tropical fish food makes sure that you mention the names of species you own to the storeowner. They will be more open to suggest the best food for your tropical fish.